1. Field of the Invention
The present invention relates to an optical transmitter, in particular, the invention relates to the optical transmitter with a function to prevent the overshooting and the undershooting in an optical output at the beginning of the feedback operation.
2. Related Prior Art
The United States patent, U.S. Pat. No. 5,978,395, has disclosed an optical transmitter for the wavelength division multiplex (WDM) optical communication. The driver circuit disclosed in this patent provides a temperature control circuit to set the temperature of the laser diode (hereinafter denoted as LD) to a value where the LD emits light with a prescribed wavelength. This feedback circuit for stabilizing the temperature of the LD utilizes an error signal, difference between the practical temperature monitored by a temperature sensor and a target temperature for the LD to emit light with the prescribed wavelength. The LD-driver in this prior art shuts off the LD when, by using the error signal, the-monitored temperature of the LD is off the target temperature. Therefore, this optical transmitter prevents to emit light when, for example just after the power-on, the temperature of the LD fluctuates.
Another Japanese patent application published as 2003-298524 has disclosed an optical source capable of stabilizing the emission wavelength. The optical source of this prior art controls in feedback the temperature of the light emitting device to be a temperature corresponding to the prescribed emission wavelength. When the optical source is powered on, the LD is shut off by the specific circuit. The control of the temperature by the feedback loop starts as the LD is kept to be shut off. After stabilizing the temperature in the target temperature, the specific circuit is disabled to operate the LD. This optical source may prevent the cross talk to the neighbor channel at the beginning of the operation for the LD.
However, the feedback control of the temperature to set the temperature of the LD to be a preset value occasionally brings an overshoot or an undershoot in the temperature of the LD, equivalently in the emission wavelength thereof, because of the high closed loop gain of the feedback control and a large time constant of elements within the loop, such as a thermoelectric controller. In the transient response using the feedback loop, the temperature of the LD finally converges in a range around the predetermined value as oscillating over this convergent range, or iterating the overshoot and the undershoot with relaxing the magnitude thereof. Therefore, when the LD is turned on under the condition that the temperature thereof is within the convergent range around the target temperature, the emission wavelength occasionally becomes out of the acceptable shift from the target value.
The present invention, carried out in the light of the above subjects, provides an optical transmitter that prevents the overshoot and undershoot in the emission wavelength exceeding the acceptable range at the transmitted being powered on.